1. Field of the Invention
The present invention relates to a vapor deposition mask, an organic electroluminescent (hereunder EL) display device, and a manufacturing method therefor. In particular the present invention relates to a vapor deposition mask in an active matrix drive organic EL display device of a top emission type, an organic EL display device, and a manufacturing method therefor.
2. Description of the Related Art
As a display device which can be made thin and lightweight as compared to conventional cathode-ray tubes (CRT) and liquid crystal displays (LCD), a display device using organic EL elements has recently attracted a great deal of attention.
Since the organic EL element is self light emitting, it has various characteristics such as, the visibility is high, there is no viewing angle dependency, a film substrate having flexibility can be used, and it is thin and lightweight as compared to the liquid crystal display.
In conventional organic EL display devices, for example an anode made from a transparent conductive film such as indium tin oxide film (hereunder ITO) is formed on an insulation substrate made from a glass substrate, and on this anode is formed an organic EL layer containing a luminous layer which generates light by recombination of electrons and positive holes. Furthermore, on the organic EL layer is formed a cathode made from Al or Mg—Ag alloy or the like. That is to say, on the insulation substrate is formed the anode, the organic EL layer, and an organic EL element comprising the cathode. Normally, the light generated in the luminous layer of the organic EL layer is emitted from the insulation substrate side, giving a so-called bottom emission type.
Furthermore, in order to realize a high definition display, advance is being made together with the organic EL element, for the development of a display device of an active matrix type having switching elements such as thin film transistors (hereunder TFT).
In the aforementioned bottom emission type organic EL display device, a switching element such as a thin film transistor which controls the drive voltage applied to the organic EL element, shuts out the light generated in the luminous layer of the organic EL layer (has an influence on the organic EL layer). Hence the aperture ratio for the generated light with respect the entire bottom surface of the device is reduced.
Therefore, recently, so that the switching element does not cause a shadow, development is progressing for a top emission type organic EL display device where the upper section common electrode is made as a transparent or semi-transparent optical transmission electrode, and light is emitted from the upper electrode side.
In this case, for example the ITO constituting the common electrode of the upper section is formed at a low film forming temperature of 100° C. or less so as to not deteriorate the organic EL layer of the lower layer. Moreover in order to maintain the optical transmission, the film thickness is made approximately 150 nm.
However, generally for transparent conductive bodies, the electrical resistance is large to become 30Ω per □ with the aforementioned construction. In the organic EL element for the current drive, a large current is necessary at the time of high intensity emission, and due to the wiring resistance and the large current of the electrical path, an excessive load is applied to a power source.
That is to say, a voltage for where a voltage drop part comprising the product of the wiring resistance and the electrical current is subtracted from a power source voltage, becomes an effective voltage for the organic EL device.
In this case, since a resistivity of a metal wiring inside the TFT substrate is 0.2 ohms per □, then for the wiring resistance and the voltage drop in the electrical path, the high resistance of the common electrode becomes a predominant factor. Since the common electrode is connected to the metal wiring inside the substrate at the outer periphery of the screen, then at the closest portion (screen outer periphery) and the remotest portion (the screen center) from the connecting locus of the common electrode, the length of the electrical path of the common electrode becomes different, so that the screen center has a high resistance load caused by the common electrode.
Moreover, when the whole screen is lit, that is to say, when all the pixels are energized, the current of each pixel becomes superimposed from the screen center towards the common electrode connection portion of the screen outer periphery. Therefore, the high resistance of the common electrode becomes multiplied so that the voltage drop of the screen center becomes greater than that for the screen outer periphery, and a difference occurs in the effective voltage of each of the organic EL elements, giving rise to brightness unevenness.
This brightness unevenness becomes a display quality loss. The larger the display is, the more conspicuous the brightness unevenness is since the area for light emission is large so that a large current is necessary and the common electrical path becomes long.
In order to solve the brightness unevenness which is apt to happen in the top emission type organic EL display device, as shown in Japanese Patent Application Laid-Open Nos. 2002-318556 and 2004-207217, a method is proposed where auxiliary wiring for the common electrode is arranged at a TFT substrate. This is explained with reference to FIG. 15.
FIG. 15 is a schematic cross-section of a related art top emission type organic EL display device. A polysilicon island shape region 94 is formed on a glass substrate 91 via an SiN film 92 and an SiO2 film 93. A gate electrode 96 made from Al is provided on the polysilicon island shape region 94 via a gate insulating film 95 made from SiO2 film. An SiO2 film 97 is provided so as to cover the whole surface, after which openings for the source-drain region are formed, and a source electrode 98 and a drain electrode 99 are formed, to thereby form a TFT as an active element.
Next, a positive type photosensitive polyimide is coated by a spin coating method, after which only the region corresponding to the source electrode 98 is exposed and developed, followed by baking. As a result, a flat insulating film 100 having contact holes 101 corresponding to the source electrode 98 is formed.
An Al film is deposited over the whole surface, after which a lower electrode 102 and auxiliary wiring 103 are formed by patterning in a predetermined shape. An SiO2 film 104 is deposited over the whole surface, after which a through hole 105 which exposes the auxiliary wiring 103, and an aperture area which exposes the lower electrode 102 are formed.
Using a vapor deposition mask which is positioned so that a non-aperture area corresponds to the auxiliary wiring 103, a hole injection layer, a hole transport layer, and a light emitting layer are vapor deposited under heat in sequence to thereby form an organic EL layer 106. The vapor deposition mask is then removed, and an upper electrode 107 made from ITO is deposited over the whole surface, and the upper electrode 107 and the auxiliary wiring 103 are electrically connected.
Finally, as with a normal organic EL device, sealing is performed in a dry nitrogen atmosphere, by means of a sealing plate 108 made from glass using a UV adhesive, to thereby complete the top emission type organic EL display device.
In this manner, the upper electrode 107 made from ITO of a high specific resistance, is connected to the auxiliary wiring 103 made from Al of a low specific resistance. As a result, the voltage drop due to the upper electrode 107 is reduced, and hence the display characteristics are greatly improved.
However, the whole screen comprises the pixel aperture area where the organic EL layer emits light, and the non pixel aperture area other than this, and if the space for the auxiliary wiring 103 and the through hole 105 which are constructed in the non pixel aperture area is to be kept large, the pixel aperture area becomes small and an aperture ratio (a ratio of the pixel aperture area with respect to the entire top surface of the device, same hereunder) is lowered so that the brightness is reduced. In the configuration of Japanese Patent Application Laid-Open No. 2004-207217, the aperture ratio is around 30%.
Furthermore, even if the non pixel aperture are is made to the minimum value of the processing limits, the higher the definition of the device, the greater an occupation proportion of the non pixel aperture area, so that the aperture ratio is decreased. Therefore the configuration in the related arts is less advantageous for obtaining high intensity of light emission.